Modern theater-quality motion picture projectors use a motor-driven sprocket wheel to pull the film intermittently through the film gate. In 24 fps (frames per second) systems the film is advanced every 41.67 ms (milliseconds) with the period of film motion preferably occupying no more than about one-half of the cycle to allow the film frame to be projected during the rest of the cycle when it is stationary. In a higher speed system used by the Showscan Film Corporation wherein the film is projected at a rate of 60 fps, each cycle lasts 16.67 ms, and only about one-half of each cycle is available for actual film movement. Accordingly, there is a demand for a film-moving apparatus which can operate reliably and at very high speed.
In a projector of the type using a motor-driven sprocket wheel, a high accelerating voltage is applied at the beginning of each cycle to rapidly accelerate the film and move it along most of the frame-spacing distance. This is followed by a decelerating voltage which rapidly decelerates the film so it is moving slowly near the final film position. A tachometer-controlled voltage may be used near the end of deceleration. After such rapid acceleration and deceleration, during which the film may have moved about 98% of the total distance moved each cycle, the film (and motor) reach a transfer location. At the transfer location a servo-controlled fine-positioning system controls motor energization to quickly move the film to its final position so that film frame can be projected during the rest of the cycle. The shutter opens at a predetermined time after the beginning of each cycle, and it is necessary that the film reach the transfer location early enough and at a low enough speed in each cycle.
Several environmental factors can affect the time required for the film to reach the transfer location and the velocity of the film at that location. If the motor is accelerated and decelerated at high voltages to rapidly move the film, then the motor temperature and power output will change after a few minutes of operation. Wear and heating of parts, and accummulating dirt can slow film acceleration. Changes in the thickness of the film stock can increase or decrease acceleration. If the film moves much too slowly, it may decelerate to a stop before reaching the transfer location, while if a film moves much too fast, it may move past the transfer location so fast that the fine-positioning system cannot operate on it. If the acceleration and deceleration rates are reduced to account for the worst-case situation, then it will require too much time to move the film to the transfer location, and a smaller portion of each cycle will be available for the projection of the film. A system which enabled rapid acceleration and deceleration to be used for rapid film positioning in each cycle, which avoided the danger that the film would not reach the transfer location within the required time or would reach it at too great a velocity, would be of considerable value for motion picture projectors and especially for those which operate at high speed.